Secondary batteries are devices capable of storing energy in chemical form and of converting into electrical energy to generate electricity when needed. The secondary batteries are also referred to as rechargeable batteries because they can be recharged repeatedly. Common secondary batteries include lead accumulators, NiCd batteries, NiMH accumulators, Li-ion batteries, Li-ion polymer batteries, and the like. When compared with disposable primary batteries, not only are the secondary batteries more economically efficient, they are also more environmentally friendly.
Secondary batteries are currently used in applications requiring low electric power, for example, equipment to start vehicles, mobile devices, tools, uninterruptible power supplies, and the like. Recently, as the development of wireless communication technologies has been leading to the popularization of mobile devices and even to the mobilization of many kinds of conventional devices, the demand for secondary batteries has been dramatically increasing. Secondary batteries are also used in environmentally friendly next-generation vehicles such as hybrid vehicles and electric vehicles to reduce the costs and weight and to increase the service life of the vehicles.
Generally, secondary batteries have a cylindrical, prismatic, or pouch shape. This is associated with a fabrication process of the secondary batteries in which an electrode assembly composed of an anode, a cathode, and a separator is mounted in a cylindrical or prismatic metal casing or a pouch-shaped casing of an aluminum laminate sheet, and in which the casing is filled with electrolyte. Because a predetermined mounting space for the electrode assembly is necessary in this process, the cylindrical, prismatic or pouch shape of the secondary batteries is a limitation in developing various shapes of mobile devices. Accordingly, there is a need for secondary batteries of a new structure that are easily adaptable in shape.
To fulfill this need, suggestions have been made to develop cable-type batteries having a very high ratio of length to cross-sectional diameter. Korean Patent Application publication No. 2005-99903 discloses a flexible battery consisting of an inner electrode, an outer electrode and an electrolyte layer interposed therebetween. However, such battery has poor flexibility. The linear batteries use a polymer electrolyte to form an electrolyte layer, but this causes difficulties in the inflow of the electrolyte into an electrode active material, thereby increasing the resistance of the batteries and deteriorating the capacity and cycle characteristics thereof.
Also, the cable-type batteries may have spaces that are ununiformly generated among a separation layer interposed between an inner electrode and an outer electrode and such electrodes in the preparation thereof, and such a space may disturb the introduction of an electrolyte solution in an outer electrode active material layer, thereby deteriorating battery performances.
In addition, in the case of cable-type batteries having a wire-form current collector, since a value of line resistance is generally higher than that of sheet resistance, the wire-form current collector may exhibit high resistance as compared with a sheet-form current collector, thereby deteriorating battery performances.